It has long been conventional to measure the relative air speed between a moving object, such as an aircraft, and the free airstream through which the aircraft is flying by means of a mechanical instrument which in effect compares the kinetic pressure exerted by the moving airstream onto a first defined area facing said airstream with respect to the static pressure exerted on a second defined area generally perpendicular to said first defined area. Typically, such a prior art system employs Pitot tubes, pneumatic tubing and pressure transducers which are exposed to the external environment and subject to not only degraded performance caused by calibration changes, but also catastrophic failures as a result of accidental breakage. Furthermore, such a prior art type of air speed measurement device physically protrudes into the airflow, with a resultant drag penalty.
The atmosphere contains many naturally occurring aerosols having a diameter on the order of from 0.1 to 10 micrometers. Examples of such aerosols are pollen and dust that naturally occur in the atmosphere. These aerosols tend to follow the motion of the atmosphere in which they are entrained and hence by observing the motion of such particles, it is possible to measure the velocity of the surrounding airstream.
Accordingly, optical techniques have also been utilized to measure wind. One such technique employed scattered sunlight as a source and cross-correlated the outputs of two radiometers having intersecting fields of view. Against localized flumes, such as the flume from a smokestack, it produced satisfactory results. A more sophisticated type of optical device measured the Doppler shift in light scattered by particles within the moving fluid to measure velocity along the system's optical axis.
There has also been used a so-called fringe laser velocimeter which measured a velocity component transverse to the instrument's line of sight by detecting the movement of aerosol particles through a layered pattern of interference fringes created by two intersecting beams of coherent laser light. Such a device has proven particularly accurate at relatively close ranges.
Another optical technique relied upon variations in refractive index moving with the wind across the line of sight of the instrument, detecting such variation by means of a so-called double-ended system having separated source and receiver.
If it is required to measure relative wind speeds in three dimensions, it would theoretically be sufficient to provide three separate systems with mutually orthogonal axes aimed at a common region of space. In practice, such an arrangement is extremely difficult to maintain in alignment and expensive to implement.
It has been proposed to utilize simultaneously two dominant colors from a single laser to form two mutually orthogonal sets of fringe planes in a common detection volume but, nevertheless, readily distinguishable from each other so that it would be possible to measure two orthogonal velocity components in the plane transverse to the velocimeter's optical axis. Alternatively, polarization or modulation by means of acousto-optic modulators may be utilized to distinguish the signals corresponding to the two components. It has also been proposed to combine two different types of velocimeters in the same system, so that two orthogonal velocity components may be measured simultaneously, one parallel to the system's main axis (by the Doppler method), and one, at right angles thereto (using the fringe method); by also using the above-mentioned two component techniques it would thus also theoretically be possible to measure simultaneously three orthogonal velocity components (one by the Doppler method, two with the fringe method).
It also has long been conventional to measure changes in altitude by means of mechanical instruments sensitive to changes in pressure of the earth's atmosphere from one elevation to another. Accordingly, it is conventional to provide an aircraft with one or more static pressure ports so that the external air pressure is exerted upon a pressure measuring diaphragm contained within the aircraft. However, significant inaccuracies may result from disturbances by the airflow in the region of the pressure port caused by icing, by air currents and turbulence, and by air compression effects or from changes in the orientation of the port relative to the airflow caused by changes in the attitude of the aircraft.
Since it is a well known law of nature that the pressure of a gas is linearly related to its density and temperature, it is also possible to compute the pressure of the air at a given elevation from measurement of other physical quantities of the air such as air density and temperature, and then to use the thus computed pressure to determine barometric altitude.
It has also been proposed to use a high power pulse laser transmitter and a receiver with range gating circuitry to look at the fluorescence or Raman scattering return signal from a localized region of the atmosphere and thus to determine concentrations of various molecular constituents in such region and, in particular, the concentration of pollutants such as NO, NO.sub.2, CO, SO.sub.2, and O.sub.3 (nitrous oxide, nitric oxide, carbon monoxide, sulphur dioxide, and ozone).
However, taken as a whole, the known prior art does not teach or suggest how the above-mentioned optical and other related techniques may be utilized to provide a compact radiation fringe velocimeter for measuring in three dimensions. To the contrary, the known prior art suggests that at least one velocity component--that parallel to the velocimeter's main axis--should be measured by the above-described Doppler method which requires that the coherency of the radiation be maintained even after it has been scattered back towards the velocimeter where it still must be combined with unscattered light from a referenced source related in frequency and coherency to the radiation impinging upon the object, thus requiring a radiation source that is coherent and which is capable of maintaining such coherency over relatively long distances. However, the known devices which satisfy such a criterion employ a CO.sub.2 laser having a relatively long wavelength that is not optimal for the relatively small aerosol particles that are naturally found in the earth's atmosphere.
Furthermore, such known prior art does not teach or suggest the considerable improvement in signal-to-noise ratio that results from utilizing a single transmit/receive lens window to focus on a single detection volume containing three sets of fringe planes oriented with respect to one another such that no two of their three respective normal vectors are either coplanar or orthogonal with respect to each other while at the same time each of the three normal vectors has a significant component in the direction of the system's main axis as well as a transverse component at right angles thereto.
Additionally, the known prior art does not teach or show how fluorescent emissions, and, in particular, how the decay characteristics of such fluorescent emissions once the fluorescent energy source has been interrupted, may be utilized to calculate air data parameters such as the relative density of a particular molecular species or, if the fluorescing molecules represent a known percentage of the atmosphere, the density, pressure and barometric altitude of the atmosphere at the elevation at which the measurement is being made.
Moreover, taken as a whole, the known prior art does not teach or suggest how air data measurements that are critical to the operation of an aircraft (such as true air speed, side slip, angle of attack, air density, air pressure, and/or barometric altitude) can be simply and reliably measured by means of an accurate and reliable measurement device free of any Pitot tubes, pressure ports or protrusions into the airstream.
Furthermore, such known prior art does not teach or suggest any reliable method for making air data measurements at a sample location at a sufficient distance from the aircraft or any physical attachments thereto that the measurement will not be subject to systemic errors of a sort that cannot always be fully compensated for such as those caused by air compression effects and airflow disturbances. The teachings and disclosures contained in the U.S. patents and the information provided by the other publications specifically cited in the two above-referenced Co-pending applications may contribute to a better understanding of the background of the present invention, as well as of its scope, function and possible manners of implementation and use; accordingly, they are hereby incorporated in their entirety by reference the same as if fully set out herein.